Normalized difference water index

Normalized Difference Water Index (NDWI) may refer to one of at least two remote sensing-derived indexes related to liquid water:

One is used to monitor changes in water content of leaves, using near-infrared (NIR) and short-wave infrared (SWIR) wavelengths, proposed by Gao in 1996:[1]

Another is used to monitor changes related to water content in water bodies, using green and NIR wavelengths, defined by McFeeters (1996):

Overview

In remote sensing, ratio image or spectral rationing are enhancement techniques in which a raster pixel from one spectral band is divided by the corresponding value in another band.[2] Both the indexes above share this same functional form; the choice of bands used is what makes them appropriate for a specific purpose.

If looking to monitor vegetation in drought affected areas, then it is advisable to use NDWI index proposed by Gao utilizing NIR and SWIR. The SWIR reflectance in this index reflects changes in both the vegetation water content and the spongy mesophyll structure in vegetation canopies. The NIR reflectance is affected by leaf internal structure and leaf dry matter content, but not by water content. The combination of the NIR with the SWIR removes variations induced by leaf internal structure and leaf dry matter content, improving the accuracy in retrieving the vegetation water content.[3]

NDWI concept as formulated by Gao combining reflectance of NIR and SWIR is more common and has wider range of application. It can be used for exploring water content at single leaf level[4] [5] as well as canopy/satellite level [6] [7] [8] [9] [10].

The range of application of NDWI (Gao, 1996) spreads from agricultural monitoring for crop irrigation[11] and pasture management [12] to forest monitoring for assessing fire risk and live fuel moisture [13] [14] [15] particularly relevant in the context of climate change.

Different SWIR bands can be used to characterize the water absorption in generalized form of NDWI as shown in eq. 1. Two major water absorption features in SWIR spectral region are centered near 1450 nm and 1950 nm while two minor absorption features are centered near 970 and 1200 nm in a living vegetation spectrum. [16] [17] Sentinel-2 MSI has two spectral bands in SWIR region: band 11 (central wavelength 1610 nm) and band 12 (central wavelength 2200 nm). Spectral band in NIR region with similar 20 m ground resolution is band 8A (central wavelength 865 nm).

Sentinel-2 NDWI for agricultural monitoring of drought and irrigation management can be constructed using either combinations:

  • band 8A (864nm) and band 11 (1610nm)
  • band 8A (864nm) and band 12 (2200nm)

Both formulations are suitable.

Sentinel-2 NDWI for waterbody detection can be constructed by using:

  • "Green" Band 3 (559nm) and "NIR" Band 8A (864nm)


McFeeters index: If looking for water bodies or change in water level (e.g. flooding), then it is advisable to use the green and NIR spectral bands[18] or green and SWIR spectral bands. Modification of normalised difference water index (MNDWI) has been suggested for improved detection of open water by replacing NIR spectral band with SWIR. [19]

Interpretation

Visual or digital interpretation of the output image/raster created is similar to NDVI:

  • -1 to 0 - Bright surface with no vegetation or water content
  • +1 - represent water content

For the second variant of the NDWI, another threshold can also be found in [20] that avoids creating false alarms in urban areas:

  • < 0.3 - Non-water
  • >= 0.3 - Water.
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References
  1. Gao. "NDWI—A normalized difference water index for remote sensing of vegetation liquid water from space." 1996. http://ceeserver.cee.cornell.edu/wdp2/cee6150/Readings/Gao_1996_RSE_58_257-266_NDWI.pdf
  2. Lillisand & Kifer
  3. Ceccato et al. 2001
  4. Ceccato et al 2001 Remote Sensing of Environment 77 (2001) 22–33
  5. Fourty & Baret 1997 On spectral estimates of fresh leaf biochemistry. International Journal of Remote Sensing, 19, 1283–1297
  6. Susan L. Ustin, Dar A. Roberts, Jorge Pinzón, Stephane Jacquemoud, Margaret Gardner, George Scheer, Claudia M. Castañeda, Alicia Palacios-Orueta, 1998 Estimating Canopy Water Content of Chaparral Shrubs Using Optical Methods, Remote Sensing of Environment,Volume 65, Issue 3,Pages 280-291,ISSN 0034-4257, https://doi.org/10.1016/S0034-4257(98)00038-8
  7. Serrano, L., Ustin, S.L., Roberts, D.A., Gamon J.A. & Peñuelas, J. 2000. Deriving water content of chaparral vegetation from AVIRIS data. Remote Sensing of Environment, 74(3):570-581.
  8. P. E. Dennison, D. A. Roberts, S. H. Peterson & J. Rechel (2005) Use of Normalized Difference Water Index for monitoring live fuel moisture, International Journal of Remote Sensing, 26:5, 1035-1042, DOI: 10.1080/0143116042000273998
  9. Serrano, J.; Shahidian, S.; da Silva J. M. (2019) Evaluation of Normalized Difference Water Index as a Tool for Monitoring Pasture Seasonal and Inter-Annual Variability in a Mediterranean Agro-Silvo-Pastoral System. Water 2019, 11, 62; doi:10.3390/w11010062
  10. Marusig, D.; Petruzzellis, F.; Tomasella, M.; Napolitano, R.; Altobelli, A.; Nardini, A. Correlation of Field-Measured and Remotely Sensed Plant Water Status as a Tool to Monitor the Risk of Drought-Induced Forest Decline. Forests 2020, 11, 77
  11. E. Farg, S. Arafat, M.S. Abd El-Wahed, A. El-Gindy, 2017 Evaluation of water distribution under pivot irrigation systems using remote sensing imagery in eastern Nile delta. https://doi.org/10.1016/j.ejrs.2016.12.001.
  12. Serrano, J.; Shahidian, S.; da Silva J. M. (2019) doi:10.3390/w11010062
  13. P. E. Dennison, D. A. Roberts, S. H. Peterson & J. Rechel (2005) DOI: 10.1080/0143116042000273998
  14. Abdollahi, M.; Islam, T.; Gupta, A.; Hassan, Q.K. An Advanced Forest Fire Danger Forecasting System: Integration of Remote Sensing and Historical Sources of Ignition Data. Remote Sens. 2018, 10, 923.
  15. Marusig, D.; Petruzzellis, F.; Tomasella, M.; Napolitano, R.; Altobelli, A.; Nardini, A. Correlation of Field-Measured and Remotely Sensed Plant Water Status as a Tool to Monitor the Risk of Drought-Induced Forest Decline. Forests 2020, 11, 77
  16. Curran, P.J. (1989) Remote Sensing of Foliar Chemistry. REMOTE SENS. ENVIRON. 30:271- 278
  17. Jacquemoud & Ustin, 2003: Application of radiative transfer models to moisture content estimation and burned land mapping http://www.ipgp.jussieu.fr/~jacquemoud/publications/jacquemoud2003.pdf
  18. S. K. McFEETERS (1996) The use of the Normalized Difference Water Index (NDWI) in the delineation of open water features, International Journal of Remote Sensing, 17:7, 1425-1432, DOI: 10.1080/01431169608948714
  19. Xu, 2006: Xu, Hanqiu "Modification of Normalised Difference Water Index (NDWI) to Enhance Open Water Features in Remotely Sensed Imagery." International Journal of Remote Sensing 27, No. 14 (2006): 3025-3033. https://doi.org/10.1080/01431160600589179
  20. https://www.mdpi.com/2072-4292/5/7/3544/htm
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